New techniques for separating mixtures of polymerizable monomers are widely sought. In the chemical industry, a high degree of purity in substances which are subjected to polymerization is necessary insofar as it permits the preparation of polymers of high molecular weight, a high reactivity of the polymerization system, a high yield of polymer, and a low consumption of catalyst. It is also necessary to control the influence of various impurities on the characteristics of the polymers. For example, as an impurity in t-butylstyrene, isopropenylstyrene is a polyfunctional monomer and acts as a cross-linking monomer when the t-butylstyrene is polymerized. For use as a polymerization monomer, it is desirable and usually specified that the t-butylstyrene contain not more than, and preferably less than, 50 ppm of isopropenylstyrene.
Various techniques have been suggested for purifying monomers, such as fractional or extractive distillation, crystallization and zone-wise fusion, extraction and treatment with compounds which react with some impurities in the monomers. Until now, there has been no acceptable way to remove small amounts of .alpha.-alkylstyrene, especially amounts below about 5000 ppm, from mixtures of styrene and .alpha.-alkylstyrene or derivatives thereof.
U.S. Pat. No. 2,215,569 teaches that mixtures of 25 percent styrene, 17-25 percent .alpha.-alkylstyrene and 50-58 percent isopropylbenzene may be treated with sulfuric acid at about 70 percent to 90 percent strength at ordinary temperatures so as to effect the polymerization of the .alpha.-alkylstyrenes leaving the styrene unchanged.
For the purpose of treating sytrene monomers containing .alpha.-alkylstyrene impurities, sulfuric acid having a concentration in a range of about 70 percent to about 90 percent removed the .alpha.-alkylstyrene impurities; however, continued contact with the sulfuric acid led to the polymerization of the desired styrenic monomer.
It is desirable to keep the concentration of impurities, such as .alpha.-alkylstyrene and nuclear alkyl derivatives thereof, to a minimum, since they will react to form undesirable branched products in polymerization mixtures with styrene and nuclear alkyl derivatives of styrene. Further, it is particularly desirable to have the desired polymerizable styrene monomer in a high degree of purity since this will enhance the economics of production, sale and use. In accordance with the present invention, separation of mixtures of polymerizable aryl-substituted mono-olefins is obtained by contacting the mixture with an appropriate ion exchange resin. This process requires very low power consumption; avoids polymerization of the desired monomer; does not modify the properties of the desired monomer; limits loss of chemicals due to successive manipulations; avoids inconveniences due to washing, disposal of a deleterious waste stream, or use of extensive heat.